1. Field of the Invention
The present invention relates to a branching filter suitable for use in communication devices such as portable telephones, and also relates to a communication device including the branching filter.
2. Description of the Related Art
In recent years, the technological progress in miniaturization and weight reduction of communication devices such as portable telephones has been remarkable. In order to realize such progress, not only reduction and miniaturization of components constituting the communication devices, but also development of composite components having plural functions has been ongoing. In particular, a high-performance branching filter used in the RF stage is required.
As a branching filter used in the RF stage as mentioned above, there is a branching filter using a ladder-type piezoelectric thin-film filter, as described in Japanese Unexamined Patent Application Publication No. 2001-24476 (publication date: Jan. 26, 2001). The ladder-type piezoelectric thin-film filter is constructed such that piezoelectric thin-film resonators functioning as a series resonator and a parallel resonator are arranged so as to define a ladder-type arrangement. The piezoelectric thin-film resonator contains a substrate having an opening portion or a concave portion and a vibrating portion in which at least a pair of an upper electrode and a lower electrode, which face each other, sandwich a thin-film portion having one or more piezoelectric thin-film layers formed on the opening portion or the concave portion.
In the filters constituting a branching filter, although dependent on applications, it is required that the attenuation on the lower-frequency side or the higher-frequency side of the passband be sharp. The branching filter contains a transmission-side filter and a reception-side filter and, although dependent on its application, there are cases in which the frequency spacing between the transmission-side filter and the reception-side filter is made very narrow. In that case, in the transmission-side filter positioned on a relatively lower-frequency side, the sharpness is required on the higher-frequency side close to the passband, and, in the reception-side filter positioned on a relatively higher-frequency side, the sharpness is required on the lower-frequency side close to the passband.
For example, in PCS in North America, the transmission band is 1850 MHz to 1990 MHz and the reception band is 1930 MHz to 1990 MHz. Both the transmission-side and reception-side passbands are as wide as 60 MHz, but the spacing (guard band) between the transmission band and the reception band is 20 MHz and this is only 1% of the carrier frequency. Since the sharpness of the filters is required to be in this range of 20 MHz, while considering the amount of frequency change due to temperature changes and manufacturing variations, filters having as sharp of characteristics as possible are required.
In the reception-side filter, a low insertion loss (for example, 3.0 dB or less) is required in the passband, and a high attenuation value (for example, 50 dB or more) is required in the other party's frequency band (passband of the transmission's side filter). Accordingly, a sharpness maintaining the attenuation value of 3.0 dB to 50 dB over 1930 MHz to 1910 MHz is required. Actually, since it is necessary to consider the amount of frequency change due to temperature changes and manufacturing variations, it is desirable that the amount of frequency change from 3.0 dB to 50 dB be about 10 MHz.
However, in the branching filter described in Japanese Unexamined Patent Application Publication No. 2001-24476, it was difficult to realize a branching filter having excellent characteristics in which a ladder-type piezoelectric thin-film filter having a wide passband and excellent sharpness outside the passband is provided.
In the ladder-type piezoelectric thin-film filter, the sharpness on the lower-frequency side close to the passband depends on the frequency spacing between the resonance frequency and the antiresonant frequency of the parallel resonator, and the sharpness on the higher-frequency side close to the passband depends on the frequency spacing between the resonance frequency and the antiresonant frequency of the series resonator. Therefore, the sharpness on the lower-frequency side and the higher-frequency side close to the passband can be improved by narrowing the frequency spacing between the resonance frequency and the antiresonant frequency of the parallel resonator or the series resonator.
The ladder-type thin-film filter described in Japanese Unexamined Patent Application Publication No. 2001-24476 is composed of piezoelectric thin-film resonators using a fundamental wave. Since the piezoelectric thin-film resonators using a fundamental wave have a large electro-mechanical coupling coefficient, it is difficult to narrow the frequency spacing between the resonance frequency and the antiresonant frequency of the piezoelectric thin-film resonators, and accordingly, it was difficult to improve the sharpness.
Furthermore, in the ladder-type piezoelectric thin-film filters, in order to obtain good characteristics in the passband, it is common to make the antiresonant frequency of the parallel resonator agree with the resonance frequency of the series resonator. However, in this case, since the passband is determined only by the frequency spacing between the resonance frequency and the antiresonant frequency of the parallel resonator and the frequency spacing between the resonance frequency and the antiresonant frequency of the series resonator, it was difficult to make the passband wider.
In particular, when, in order to obtain excellent sharpness outside the passband, the frequency spacing between the resonance frequency and the antiresonant frequency of the parallel resonator or the series resonator is narrowed and, in order to obtain excellent characteristics in the passband, the antiresonant frequency of the parallel resonator is made to agree with the resonance frequency of the series resonator, the passband is narrowed. In contrast with this, when, in order to widen the passband, the frequency spacing between the resonance frequency and the antiresonant frequency of the parallel resonator or the series resonator is widened and, in order to obtain excellent characteristics in the passband, the antiresonant frequency of the parallel resonator is made to agree with the resonance frequency of the series resonator, it becomes difficult to obtain excellent sharpness outside the passband.